Although CRTN recognises that sound absorptive noise barriers will reduce reflection effects, no allowance is given in the method to take this into account.
DMRB 2008 at subsequent editions, advised that the potential benefits of absorptive barriers should be estimated from the reflection correction as described in para 26.2 or 36 of CRTN. Put simply, the barrier reflection (typically, +1.5 dB) should be removed when an absorptive barrier is used.
Since this is an 'additional procedure' for CRTN, NoiseMap does not implement it by default. You have to choose the DMRB 2008 calculation procedures option from the NoiseMap's Calculation Parameters menu.
Note that when you choose this option, the other DMRB additional procedures are also invoked.
